This invention relates to a hydraulic brake force multiplying device of the type which is connected directly to a master cylinder in a brake system of an automobile or the like, and wherein when fluid pressure from a source of pressure fluid (a power pump, etc.) is in normal condition, the device is adapted to operate as a so-called non-stroke type device (stroke of an input shaft is determined merely so as to open or close a valve mechanism involved in the device, irrespective of the stroke of the master cylinder), and wherein when pressure source fails the device is adapted to directly actuate the master cylinder.